centralauth: Use hiera to allow variable customization

Configure hiera to allow class parameters to be modified based on realm
(vagrant, labs, ...) and/or unmanaged local configuration files. Although this
becomes a general feature for MediaWiki-Vagrant, the initial use case is
enabling use of the centralauth role on a labs_vagrant instance.

When applied on a labs host, multiwiki will now setup a virtual host as
"*-$(hostname).wmflabs.org" rather than the "*.wiki.local.wmftest.net"
vhost created by default when running under Vagrant. The user will need
to manually setup web proxies in wikitech to make these vhosts
accessible.

The naming pattern can be changed by creating hiera data file at
/vagrant/puppet/hieradata/local.yaml which sets the
'multiwiki::base_domain' key to the suffix to expect. For example:

    multiwiki::base_domain: "-sulfinalization.wmflabs.org"

Bug: 67331
Change-Id: I87e80f28492c46122845ba3793637b8ea5e094a4